Abstract
The change in axial ratio c/a of the CuAu L10-type ordered alloy with the addition of the third element X was examined. All the ternary alloys prepared were constructed by substituting X for corresponding amount of Au in the CuAu, where X was Ni and Ag. Further, Cu-Au binary system was picked up. The results obtained by X-ray diffraction method showed that, the axial ratio increased with composition y, fairly rapidly for the CuAu1−yNiy and gradually for the CuAu1−yAgy. In the Cu-Au system, it increased with deviation of composition from stoichiometry.
The experimental results, together with those published before for the CuAu1−yPdy, were analized taking into account of two energy terms; the repulsive interaction acting between ion cores and the electrostatic energy of ions occupying lattice sites. They were expressed as a function of the axial ratio by applying the Bragg-Williams method. The numerical results of c/a under the assumption of maximum degree of order S explained qualitatively the observation in the Cu-Au and CuAu1−yPdy but failed for the CuAu1−yNiy and CuAu1−yAgy. For the latter two systems, however, using the values of S obtained from X-ray intensity measurements, the recalculated results reproduced well the observed facts.
